1. Field of the Invention
The present invention generally relates to a silicon substrate processing method for the observation of defects in semiconductor devices and a defect-detecting method.
2. Description of the Related Art
When a VLSI silicon device having a semiconductor integrated circuit formed on a silicon substrate fails, it is required to detect the defects causing the failure and examine the nature of those defects.
To observe defects, a transmission electron microscope (TEM) may be used. For the TEM observation, preparation of a very thin sample through which an electron beam can penetrate is required. For example, for the TEM observation using an electron beam with an accelerating voltage of 200 kV, the thickness of the sample to be observed should be around 0.1 μm. Some methods using a focused ion beam (FIB) to cut out a defect-containing area have been proposed for the preparation of samples used for the TEM observation (Patent documents 1 through 3).
Before preparing such a very thin sample, it is necessary to identify the specific location of a defect in the silicon substrate or the metal layer formed on the front side of the substrate. The present invention relates to a method for identifying specific locations of defects.
Known defect-detecting techniques include the optical beam induced current (OBIC) technique, the optical beam induced resistance change (OBIRCH) technique, and the photoemission microscopy (PEMS) technique.
In the OBIC and OBIRCH techniques, a constant voltage is applied to a target device, the observation point in the device is scanned and irradiated with a laser beam, and each electric current variation is displayed as a brightness signal variation on a point on the screen which corresponds to each point in the scanned area. The OBIC technique uses a visible laser (with a wavelength of 623.8 nm, for example) to detect electric current variations in a silicon substrate. The OBIRCH technique uses a visible laser (with a wavelength of 623.8 nm, for example) or a near-infrared laser (with a wavelength of 1300 nm) to detect resistance variations in a metal layer which are caused by temperature rise.
The PEMS technique uses luminescence which is seen when a voltage is applied to a target device. If the device has a defect such as junction leakage or a damaged insulator film, the electric field is concentrated near the defect and hot carriers are generated. The hot carriers emit light when they recombine. The PEMS technique detects this light to identify the location of the defect.
Detection of defects may be performed either from the front side or the backside of a silicon device. However, in defect detection from the front side, emission leakage through the metal layer may prevent accurate detection of defects. In defect detection from the backside, the surface pattern is recognized through infrared ray observation, the surface pattern is scanned (using CAD navigation, for example), and the area to be processed is determined. Also, to open the front side of the device, the backside opening should be closed to maintain the mechanical strength of the sample. Therefore, after opening the front side, it is difficult to electrically detect defects from the backside.
In defect detection from the backside, if the thickness of the silicon substrate is 15 μm or more, the resolution and the location accuracy become low. Furthermore, even if a defect is detected, the thicker the silicon substrate, the longer it takes to cut out a sample from the backside for the TEM observation.
[Patent document 1] Japanese Patent No. 3485707
[Patent document 2] Japanese Patent Application Publication No. 2001-217290
[Patent document 3] Japanese Patent Application Publication No. 2004-228076
To detect defects from the backside of a silicon substrate and to improve the accuracy of location identification, it is necessary to create a thin silicon substrate.
A thinner silicon substrate is better for highly-accurate detection of defects. Normally, diffusion layers are formed on the front side of a silicon substrate, for which a well is deeper than other types of diffusion layers and has a depth of 1.5 to 2.0 μm. Since defects in a substrate are normally found in the diffusion layers, it is not appropriate to make a silicon substrate thinner than the depth of diffusion layers. Consequently, the most suitable thickness of a silicon substrate when detecting defects is 2 to 5 μm.
One way to reduce the thickness of the backside of a silicon substrate is to evenly grind the entire area. However, if the entire silicon substrate is very thin, less than 10 μm for example, the mechanical strength of the substrate becomes low and handling of the substrate becomes very difficult.
Another way is to reduce the thickness of a defect-containing area only. For this partial processing, a laser beam or a FIB may be used. In this method, the remaining thickness of the defect-containing area should be measured. Since the infrared ray can penetrate a silicon substrate, the thickness can be measured by focusing an infrared microscope on the front side and backside of the processed silicon substrate. However, the accuracy of this method is low and it is difficult to precisely measure a very thin silicon substrate with a thickness less than 10 μm.